I have always been fascinated by illusion, magic stuff and all the brain tricks witch modify the eye perception.

Later, I have discovered "the Futuroscope", Theme park near Poitiers and the visual depth. In the early 90', theses methods where experimental and also very expensive .

I have tried to make a clear synthesys on this subject and compiled many texts from the web.

These excellent web sites are of course, listed on the page "Stereolinks"

Introduction The very first publicly shown short 3D-Movie (lasting only about a minute) was made by the Lumière brothers in 1903 ("L'Arrivée du Train"), showing the arrival of a train in a railway station. It was presented at the World Fair of 1903 in Paris. It could only be viewed by one person at a time on a modified stereoscope, as a proper screening-process to divide the left and right pictures for viewing had not been invented. The first screening of 3D short motion pictures, for a paying audience, dates back to June 10, 1915, when the short Jim, the Penman was shown at the Astor Theatre New York, starring John Mason and Marie Doro along with some scenes from rural America and the Niagara Falls. They were also the very first 3-D movies in which the audience had to wear red/green anaglyph spectacles. In September of 1922, the movie "Power of Love" was released. This film featured the "anaglyph process" which involved simultaneously shooting two view of a scene and then printing the film in two different colors and combining them with layered film on one reel. The moviegoer then viewed the film wearing a special pair of glasses with one red lens and one green lens. The red lens would draw the viewers attention to the green view of the scene and the green lens would draw the other eye towards the red view of the scene. This would cause an "overlap" which made certain objects appear closer than they were and others seem to move out of the screen towards the viewer.   Experimental or novelty 3D-Films continued to be produced sporadically through the early days of cinema. The early 1950's saw Hollywood in a lot of trouble. In addition to the repercussions from Joe McCarthy's communist witchhunt amongst actors, writers and directors, the movie industry had to contend with the growing success of television. As a result, ticket sales were miserable and studio executives eagerly searched for a gimmick to get patrons to return to the theaters. The gimmick that emerged was the three-dimensional movie.   3D and Stereoscopy Perspective is often confused with 3D, which is not quite true, because the third dimension (Depth) is only "simulated". Therefore, 2 1/2-D would be a more appropriate expression. Stereoscopic imaging (or "real 3D"), however, requires a minimum of two pictures, simulating our two eyes. This can either be accomplished by using traditional photography (stereo photography), computers (for example Virtual Reality) or Lasers (Holography).   Visual Depth   The way humans perceive 3 Dimensions (visual depth) is through the use of both eyes. Each eye sees a slightly offset view of a scene. If you alternate closing your right eye then your left eye (back and forth), you will notice that the objects before you shift position slightly (left to right). The shift in position largely depends on how near or far the objects are from your eyes. This is because each eye sees from its individual vantage point. The two slightly different views are fused together by our brain in a complex way that creates our visual perception of depth.   How do the 3D Glasses work ?   In order to simulate depth when playing video games or watching a movie, each of your eyes must see a slightly different image . The two different Right Eye and Left Eye images are presented together via the odd and even horizontal lines of your TV or PC monitor. Without the glasses, you would see a blurry double view of both images, one on top of the other.  When you view 3D content using our patented 3D glasses, the left and right images are seen clearly, one eye at a time. The way that this is achieved is by rapidly alternating the opening and closing of an LCD (liquid crystal display) lens in front of each eye. While your right eye sees the right image, the left eye is blocked by a darkened LCD lens (or shutter) and vice versa, back and forth. This alternating of images occurs many times a second and your brain fuses these separate images into one truly 3-Dimensional image. The speed of the shutters is set in direct proportion to the refresh rate of your TV or computer monitor. The wired 3D glasses remain in-sync with the image source via a connecting wire to the control box. The wireless 3D glasses accomplish this by receiving an infared signal from the control box.     What is "Stereo" or "3D" ?   The word "stereo" originates from the Greek and means "relating to space". Today, when we talk about stereo, we usually refer to stereophonic sound. Originally, the term was associated with stereoscopic pictures, which were either drawn or photographed. In order to avoid confusion with stereophonic sound, one now often talks about 3D pictures and especially 3D-film, where 3D, of course, stands for three-dimensional.   A person lives in a three-dimensional, spatial, environment. Without a feeling for space, we can not move within it. Our perception of space is created almost exclusively by our eyes. There are many ways to orient oneself in space, e.g., by perspective, gradation of color, contrast and movement.   The lenses of the eyes in a healthy human being project two slightly different pictures onto the retinas, which are then transformed, by the brain, into a spatial representation. The actual stereoscopic spatial observation is a result of this perception through both eyes.   A number of otherwise healthy two-eyed people, however, have eye-defects since birth, that make stereoscopic viewing impossible. As babies, they have, in the literal sense of the word, learned to "grasp" the world. They safely orient themselves in their environment by employing one of the other above mentioned methods. Even a person with only one eye learns how to move around safely, using non stereoscopic cues. The normal picture on paper or film is only one-eyed. It is photographed with only one lens and can, therefore, not convey a true spatial perception. It is only a flat picture. But we do not have to abstain from the known natural effect. By taking two lenses and imitating the eyes, we can create such a space image.   When we examine with or without optical instruments a stereo picture created in such a manner, we form a similar perception of space in our mind.   The two necessary, somewhat different, single views can be generated by different methods. We can produce them like the old stereo artists did, first draw one, then the other single view. We may also take the exposure one after the other with a normal single lens camera. It is evident that the subject must not move during this procedure, otherwise the two pictures would be too different. A better approach is to imitate the head and mount both lenses in a common chassis. Now we have a true stereo camera. Basically it is only the joining of two mono-cameras. It is also possible to take stereo pictures with two coupled cameras. The two lenses can also be combined as interchangeable stereo optics in a single camera.   3D-Photography duplicates the way we view a 3D object or scene by taking a pair of photographs separated by a distance equal to the separation between a typical person's eyes. The two pictures then have a viewpoint similar to the view seen by the left and right eye. These images, if directed to the left and right eyes, are fused by the brain into a single image with the appearance of depth. Perhaps the most well-known example of this is the View-Master™ many of us have played with as children (of all ages).   Stereoscopy Science and technology dealing with two-dimensional drawings or photographs that when viewed by both eyes appear to exist in three dimensions in space. A popular term for stereoscopy is 3D. Stereoscopic pictures are produced in pairs, the members of a pair showing the same scene or object from slightly different angles that correspond to the angles of vision of the two eyes of a person looking at the object itself. Stereoscopy is possible only because of binocular vision, which requires that the left-eye view and the right-eye view of an object be perceived from different angles. In the brain the separate perceptions of the eyes are combined and interpreted in terms of depth, of different distances to points and objects seen. Stereoscopic pictures are viewed by some means that presents the right-eye image to the right eye and the left-eye image to the left. An experienced observer of stereopairs may be able to achieve the proper focus and convergence without special viewing equipment (e.g., a stereoscope); ordinarily, however, some device is used that allows each eye to see only the appropriate picture of the pair. To produce a three-dimensional effect in motion pictures, various systems have been employed, all involving simultaneous projection on the screen of left- and right-eye images distinguished by, for example, different colour or polarization and the use by the audience of binocular viewing filters to perceive the images properly. In holography the two eyes see two reconstructed images (light-interference patterns) as if viewing the imaged object normally, at slightly different angles. Stereoscopic Photography Many of the landscape photographers also took stereographs. These double pictures, taken after 1856 with twin-lens cameras, produce a remarkable effect of three dimensions when viewed through a stereoscope. Stereography, first described in 1832 by the English physicist Charles Wheatstone, is uniquely photographic, since no artist could draw two scenes in exact perspective from viewpoints separated only 2? inches - the normal distance between human eyes. Wheatstone's mirror stereoscope, however, was not practical for use with photographs, and the invention languished until the Scottish scientist Sir David Brewster designed a simplified viewing instrument, which was exhibited at the 1851 Great Exhibition in the Crystal Palace, London. Queen Victoria was entranced by the stereo daguerreotypes she saw there, and with the introduction of the collodion process, which simplified exposure and printing techniques, three-dimensional photography became a popular craze.   In 1854 the London Stereoscopic Company was formed. Their chief photographer was William England, whose lively street scenes of New York City in rainy weather and views of Niagara Falls taken in 1859 were the wonders of the day. The instantaneous street scenes, which showed pedestrians and vehicles stopped in their tracks, were made possible because the small size of the stereo-camera reduced exposure times to less than half a second. To minimize movement street views were usually taken from a first-floor window with the camera focused directly down the street. (Such views later inspired several Impressionists to paint similar street scenes.) Between 1860 and about 1920 a stereo viewer was as ubiquitous in British and American homes (where a simplified and cheap hand viewer was introduced by Oliver Wendell Holmes [the American physician was a great lover of photography]) as the television set is today. Millions of stereographs were circulated in the years before newspaper reproduction of photographs, and their impact was enormous.       Chromostereoscopy Invented by Richard Steenblik ([STEENBLIK87]) as a way to amplify the common chromostereoscopy phenomenon into a useful display tool. ChromaDepth consists of two pieces: a simple pair of glasses and a display methodology. The glasses contain very thin diffractive optics that have the efficiency of refractive optics. While being very thin and inexpensive 2 , they behave like thicker glass prisms. The optics are designed so that red light is bent more than green and green more than blue. The lenses are oriented sideways, so the overall bending effect looks like parts of the scene have been shifted horizontally inwards (ie, towards the center of your nose). The red hues are shifted more than the greens and the greens are shifted more than the blues. Thus, red elements in the 3D scene appear to converge closest to the viewer and the blue elements appear to converge the farthest away.   Chromostereoscopy is a technique for converting color into stereoscopic depth. This phenomenon has been known for more than 100 years. Special glasses containing high-tech blazed gratings amplify this effect. These glasses enable the creation of 'normal' looking color images that can be viewed as two-dimensional images without glasses, but which jump into 3-D when viewed through the glasses. The physiological and physical background will be explained. Simple experiments will be shown with this inexpensive and easily obtainable device. The human eye has a strong chromatic aberration. Between far red and deep blue there is a difference of about 2 dpt. If you look at a point light source that emits only red (750nm) and blue (400nm) light, you cannot see both colors simultaneously sharp. Normally you will see a red point and a blurred blue disc. This is the so-called longitudinal chromatic aberration. Blue light is refracted more than red light (chromatic dispersion of the eye media). Furthermore, the red point is not centered in the blue disc. This is due to the transversal chromatic aberration and occurs because the line of sight (line between the point source and the fovea; see Fig. 1, left side) does not coincide with the optical axis of the eye. With a simple experiment you can verify the longitudinal aberration. Through a cobalt glass filter look at a point light source that emits enough intensity even at the ends of the visible spectrum. Cobalt filters absorb almost all of the visible light. Only red and blue light can pass. You can see this with one eye or with both eyes open. The longitudinal chromatic aberration has been known for long. It is used regularly by ophthalmologists to test visual acuity (red-green test). It is known also in the advertising industry. For example, red letters on a blue background must be avoided; otherwise, the accommodation of the eye will change between both colors, and the letters will appear unsharp and unstable. The transversal chromatic aberration is difficult to see directly, but with two eyes you can see the effect that it causes. Imagine a red-blue point light source at a distance of several meters. In reality red and blue characters printed on a black background or seen on a monitor display work much better. The blue and red light is refracted differently in the eye media. Imagine looking at the blue point. This means that the image of the blue point is directly on the fovea. Since red light is not refracted as strongly, the image of the red point is located in both eyes, a little to the side of the blue image on the temporal side. Our brain interprets this as if there are two light sources at different distances. The red source seems to be closer. The effect is small, and many people are never aware of it. If you know about the phenomenon, you may see it. The dashed lines in Figure 1 leads to the apparent image positions. This effect has been known as chromostereoscopy or color-stereo effect [1] for more than a hundred years. I have to mention here that the opposite effect is also possible: blue can appear closer than red. This depends upon the position of the fovea relative to the point of intersection of the optical axis with the retina.   Optical Separation Devices Left and right eye images are presented side-by-side with some sort of optical device used to channel the proper image into the proper eye. Many systems work this way, from mirrors that mount to monitor faces to stereo slide viewers to ViewMastersTM to sophisticated virtual reality display devices. Some people can freeview side-byside stereo views without any special equipment (in either a parallel or cross-eye viewing mode), but this is not common in the general population. Red/Blue Anaglyph Left and right eye images are combined into a single image consisting of blues for the left eye portion of the scene, reds for the right eye portion of the scene, and shades of magenta for portions of the scene occupied by both images. The viewer wears a pair of glasses with red over the left eye and blue over the right eye. Each eyepiece causes linework destined for the other eye to meld into the background and causes linework destined for its own eye to appear black. Many people's first experience with stereo vision was using this technique while watching the classic movie Creature from the Black Lagoon. Unfortunately, the anaglyphic process could not accomodate full color movies and often caused viewers to suffer from headaches. This led to the development of the Polaroid 3D system which used two lenses filming, involved lightwaves passing in perpendicular planes to the other lens. It was this process that was used in "Bwana Devil". On November 26, 1952, the low budget independent feature film called "Bwana Devil", produced by Sidney W. Pink, opened to sold out crowds with the line of people waiting to get in spanning several blocks. The film, centered on an attack on railroad crews by man-eating lions proved so successful that United Artists purchased the rights for the film and released it nationally.  A year later, the movie "House of Wax" was released starring Vincent Price and Charles Bronson. Considered the finest 3D movie ever made, House of Wax caused a "3D" craze throughout Hollywood, with most major studios rushing to show their attempt at the novelty including "Creature from the Black Lagoon", "The Nebraskan" and "Kiss Me Kate". Unfortunately, even the prospect of Jane Mansfield's ample bosom being thrust out towards the audience was not enough to continue the craze. Still mired by a propensity to cause headaches, 3D movies fell out of favor so much that two-dimensional versions often significantly outearned the 3D version. The public rebuke was such that Alfred Hitchcock's "Dial M for Murder", originally filmed in 3D, was released only in 2D. The money quickly and eagerly thrown at participating in the just as quickly went down the drain. Today many IMAX films are made in 3D.   Polarized Lenses Left and right views are projected through orthogonal polarizing filters into a single image, which is the viewed through polarizing lenses. Highly informative stereo slide presentations can be done this way, as well as movie and video presentations. This is also the basis for Disney's stereo movies Captain Eo and Honey, I Shrunk the Audience. CrystalEyesTM: this variation of the polarizing lenses is the most common of the singlemonitor computer graphics stereo display methods. CrystalEyes displays the left and right eye views of a synthetic scene in sequential refresh scans of a monitor and then uses synchronized polarized shutter glasses to channel the correct image into the correct eye. This is also the basis for the stereographics in the CAVE virtual reality display environment ([CRUZ-NEIRA93]). Other techniques have also been used for stereoviewing, such as lenticular displays, random dot stereograms, and the Pulfrich effect. There are surely others. As these are less relevant to interactive and published scientific visualization, they were not covered here. Limitations of Existing Methods These methods all work reasonably well for limited uses. But, they all have problems when used for serious interactive and publication scientific visualization: •Some methods (polarized or CrystalEyesTM) cannot be reproduced in print because their stereo effect is tied to their display method. This means that they cannot be used for stereo presentation in papers, articles, or on web pages. • Other Methods (anaglyph for example)create an image that is unrecognizeable unless the viewer is wearing the proper glasses.   Holography Holography (from the Greek, holos whole + graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. Overview Holography was invented in 1947 by Hungarian physicist Dennis Gabor (1900-1979), for which he received the Nobel Prize in physics in 1971. The discovery was a chance result of research into improving electron microscopes at the British ThomsonHouston Company, but the field did not really advance until the invention of the laser in 1960. Various different types of hologram can be made. One of the more common types is the white-light hologram, which does not require a laser to reconstruct the image and can be viewed in normal daylight. These types of holograms are often used on credit cards as security features. One of the most dramatic advances in the short history of the technology has been the mass production of laser diodes. These compact, solid state lasers are beginning to replace the large gas lasers previously required to make holograms. Best of all they are much cheaper than their counterpart gas lasers. Due to the decrease in costs, more people are making holograms in their homes as a hobby.     Pulfrich Effect   Get a buddy to drive a car about 10 MPH along a suburban neighborhood where you have things (trees, fences, houses, etc.) both near the road and far away from the road. Your job is to sit in the front passenger seat, hold the recording video camera steady, and just point it out the right window, perpendicular to the direction of travel. Then rush back home and watch the video with only the right eye covered by sunglasses (You can also use polarized flip-down shades, but polarization has nothing to do with it) and you have Pulfrich 3D!     Lenticular Imaging    The lenticular image combines two components. The first is a plastic sheet (or glass plate), incorporating a series of parallel lenses that magnify portions of a segmented image that is printed or otherwise attached to the back (second surface) of the lens sheet. The second component, a segmented image on the second surface, is composed of alternating slices of two or more original images. The slices are interlaced (combined in alternating patterns) in such a way that, when viewed from one angle, the sheet displays a visually seamless version of one of the originals, but viewed from another angle, we see a different image. Lenticular Imaging is not exclusively used for Stereoscopic Imaging. A number of other applications exist as well. However, each of them uses the same basic principle as described in this section of the FAQ. For completeness, we mention all applications including, of course, 3D.    ColorCode ColorCode 3D (TM) is a new danish state of the art of stereo image technology. A stereo pair can be colorcode 3D encoded for computer monitors , digital projectors, films, inkjet prints... At a first glance, the ColorCode wViewer with its Amber-Blue filters may remind you of the red-blue Anaglyphs. However, this state-of-the-art 3-D stereo technology is entirely different and the images are in full color. In essence, the color information is conveyed trough the amber filter and the parallax information -to perceive depth- is conveyed through the blue filter (www.colorcode3d.com)